The invention relates to flux or paste compositions useful for brazing aluminum materials to be jointed. The term "aluminum" referred to herein and in the appended claims means aluminum and its alloys otherwise specifically described.
In order to braze aluminum, brazing sheets are frequently used. The brazing sheets used comprise cores of alloys such as 3003 alloy, 3N03 alloy or the like clad with brazing materials such as 4343 alloy or 4045 alloy on either or both surfaces thereof. These numbers of four ciphers are as specified in JIS.
In case aluminum is brazed, it is essential to coat a flux to an area to be brazed. Generally, a suspension comprising a flux in a medium such as water is coated by spraying or using any other means and dried before a brazing step. However, the spraying of the suspension may scatter the flux over undesirable areas, which is not preferable to operating atmosphere and health. When water is used as the medium, the flux is readily peeled off and dropped after completely drying and therefore, it is impossible to store and transport for a long period before the brazing step. And, processing, assembly and the like are also impossible. On the other hand, when the drying before the brazing step is incomplete, water content in a brazed area is increased which may lower a brazing property and produce defects in the brazed area. Moreover, when water is used as the medium, the flux is not well adhered to the brazing sheet. And, it is not evenly coated. Excess flux in areas to which much amounts of the flux are adhered may form gray or white stains, while areas to which less amounts of the flux are adhered may have an insufficient brazing property.
For overcoming the problems mentioned above, a method comprising adding a water soluble resin as a binder was proposed as disclosed in JP 6285682A. This method is not commercially valuable since the reaction of the binder with the flux during brazing makes brazed areas dark. Even if the water soluble resin was used, the resultant film was readily peeled off due to its poor film-forming property. Processing and assembly after the coating of the flux were substantially impossible.